In a wireless communication, a frequency is a very limited resource. For example, in Japan, TV broadcasts are carried out in 470 to 770-MHz bands, but time zones where no communication is performed may exist in the other frequency bands or even in the above bands.
Such frequency bands that are in a temporary or continuous non-use state are referred to as a white space. Research and development, and standardization have been conducted on wireless communication systems, such as IEEE 802.11af, IEEE 802.15.4m, and IEEE 802.22, so as to allow secondary users to utilize the white space.
In a case where the wireless communication is performed using the white space, it is necessary to comply with the spectrum mask regulation of each country, such as the transmission spectrum mask regulation of the Federal Communications Commission (FCC), so as to prevent disturbance in usage by a primary user. The transmission spectrum regulation of the wireless communication in the white space is stricter than the transmission spectrum mask such as a 2.4-GHz-band wireless LAN. Among them, the FCC is particularly strict.
Meanwhile, in the 2.4-GHz-band wireless LAN, a wireless communication using an Orthogonal Frequency Division Multiplexing (OFDM) scheme is used (refer to Patent Literature 1).
The OFDM scheme is a communication scheme that has various advantages, such as strong resistance to multipath transmission channels or high frequency utilization efficiency. In the OFDM scheme, data are mapped to a plurality of subcarriers aligned to be orthogonal to one another, a time-series signal is derived by performing an inverse Fourier transform on the data group, and a transmission symbol for wireless transmission is generated. Then, on the receiving side, the transmitted data group is derived by performing Fourier transform on the symbol, and the transmission data is reproduced by restoring individual data associated with each subcarrier.
Also, in addition to the OFDM, Orthogonal Frequency Division Multiple Access (OFDMA) also has been put to practical use. The OFDMA is a multiple access scheme in which a plurality of terminals transmits signals through different subcarriers.
In a wireless communication apparatus using the OFDM or the OFDMA, various methods of suppressing out-of-band radiation are used in order to satisfy the transmission spectrum mask of each country and each system.
Examples of the method of suppressing the out-of-band radiation may include a method of performing waveform shaping by using a windowing (refer to Non Patent Literature 1), a method of applying a convolution filter to a transmission signal on a baseband (refer to Non Patent Literature 2), and a method of reducing signal transmission power so as to comply with a transmission spectrum mask regulation.
In any case, it can be said that the transmission spectrum mask is defined by a signal obtained after a radio frequency conversion of a baseband signal, that is, a baseband signal waveform, and transmission spectrum characteristics of a function or an apparatus that performs radio frequency conversion.